Quest for Travel to Hong Kong

I have a Chinese passport and US green card and is currently living in the states. I need to enter Hong Kong from China in the next two months. Does anyone know whether I need to apply for a visa? and where I need to apply to? I know I can travel to China via Hong Kong without problem. But this time I will travel with a couple of others and my schedule only allows me to go to Beijing first.
Thanks for any help in advance.